[Espeefan]

I too would suggest looking at the planetary style gearmotors. You might even consider robbing such a set-up from a cheap power screwdriver, or an old cordless drill. You could run a rear axle, using a driveshaft with that set-up, or you could also drive the wheels directly off the output shaft of the gearmotor, although then you will one motor for each wheel you wish to power.